The present invention relates generally to a method and apparatus for electro-treating a metal strip and more particularly to a method and apparatus for electro-treating either one surface or both surfaces of the strip in a continuous operation.
An example of electro-treating is the electrolytic plating of a surface of a metal strip, e.g., electrolytic galvanizing wherein a steel strip is plated with zinc. Other examples of electro-treating include the electrolytic cleaning or pickling of a surface of the metal strip.
In a typical electro-treating process, a continuous metal strip is passed through a bath of electrolytic liquid contained in a tank. Electrical charges are provided so that the metal strip constitutes one electrode in an electrolytic cell with another electrode being located in contact with the electrolytic liquid. An electric current flows through the electrolytic liquid between the metal strip and the other electrode, and, depending upon whether the metal strip is to be plated or cleaned, ions flow to or from a strip surface to be either deposited thereon or removed therefrom.
For example, in an electro-galvanizing operation, the metal strip is provided with a negative charge, so as to be a cathode, metallic anodes are placed in adjacent relation to the metal strip, and the electrolytic liquid contains zinc ions. The anodes may be depletable, in which case they are composed of zinc.
If both surfaces of the metal strip are to be coated with zinc, a zinc anode is placed alongside each surface of the metal strip, adjacent thereto. If only one surface of the metal strip is to be coated with zinc, a zinc anode is placed alongside only one surface of the metal strip, and an attempt is made to prevent zinc cation's from depositing on the other surface. In most one-sided electro-galvanizing operations there is a tendency for cation's to deposit on the side edges of the strip and adjacent the side edges on the strip surface opposite that which is alone intended to be plated.
A number of procedures have been employed in the past to produce a one-sided electro-galvanized coating. One such procedure employs a horizontally disposed roller covered with a non-conductive material such as rubber. The metal strip is wrapped around a substantial portion of that roller. The inner surface on the wrapped portion of the strip is in close contacting engagement with the roller's rubber outer layer, and the outer surface of the strip is exposed. The roller is located within the bath of electrolytic liquid. Also located within the bath of electrolytic liquid, and spaced a very short distance from the outside surface of the rubber-coated roller and from the strip in an arcuate-shaped anode of zinc which is concentric with the outside surface of the roller. A multiplicity of such roller-arcuate anode arrangements (e.g., 20 to 40) are located in series in the tank.
The strip is advanced in a downstream direction, and the rollers are rotated. When current is applied to the electrolytic cell composed of the metal strip, the electrolytic liquid and the arcuate anodes, zinc is deposited from the anodes onto the exposed surface of the metal strip, while the inner surface of the metal strip, in close contacting engagement with the rubber outer layer on the roller, is, for the most part, protected against the deposition of zinc cations thereon.
However, even with the procedure described in the preceding paragraph, there is some deposition of zinc along the side edges of the strip and adjacent the side edges of the strip on the strip surface opposite that which is alone intended to be coated.
It is sometimes desirable to differentially coat a metal strip with different amounts of coating metal on each of the two surfaces of the strip. Employing the abovedescribed prior art procedure for electro-coating only one strip surface at a time, a complicated arrangement must be utilized in order to turn the strip around so that the surface which was previously exposed to the electrolytic liquid is now pressed against the rubber outer layer on the roll and the surface which was previously protected from the electrolytic liquid is now exposed thereto.
The rate with which coating metal is deposited on a metal surface is directly proportional to the current density in the electrolytic cell. Current density is expressed as amperes per unit area of metal surface undergoing coating. It is desirable to maximize the current density for a given voltage applied to an electrolytic cell. For a given voltage, current density decreases with an increase in electrical resistance along the path in the electrolytic liquid along which the cations must pass before they can be deposited upon the surface undergoing coating. Resistance increases with an increase in the distance the cations must travel before they are deposited upon a surface. A typical voltage applied to an electrolytic continuous galvanizing cell is about 15 volts. For such a voltage, the current density usually obtained in prior art continuous electrolytic galvanizing operations is in the range 500-1,000 amps./ft..sup.2 (5,380-10,760 amps./m.sup.2), at best. In one-sided galvanizing operations of the type described above, where the anode is depletable, the distance which the cations must travel increases as the anode depletes, thereby increasing the resistance and decreasing the current density as the process continues.